This invention relates to distillation systems and, more specifically, to an improved, highly efficient, rotary evaporator and condenser for use in a vapor compression distiller.
Distillation is a common method for generating potable water from otherwise unsafe water sources (such as sea water or polluted ground water). With distillation, water is heated to boiling, and the resultant vapor (i.e., steam) is collected and condensed, producing distilled water. Many contaminants that are present in the water source are left behind when the water is converted to its vapor phase. Conventional small distillers typically incorporate an electric heating element to boil water in a tank. A condensing coil mounted above the tank collects the vapor and condenses it. The distilled water is then transferred to a holding tank or cell. These boiler-type distillers, however, require substantial amounts of electrical power to produce relatively little distilled water and are thus highly inefficient. They are also extremely slow, often taking many hours to produce just a few gallons of distilled water. Accordingly, boiling-type distillers have not gained widespread acceptance or use.
In addition to boiler-type distillers, thin-film distillers have also been proposed. For example, U.S. Pat. No. 4,402,793 to Petrek et al. titled MULTIPLE EFFECT THIN FILM DISTILLATION SYSTEM AND PROCESS is directed to a solar-powered, thin film distiller. In the distiller of the '793 patent, a plurality of parallel, spaced-apart plates are arranged to face the sun. Water to be distilled is supplied to the tops of the plates and guided to run down the back face of each plate. Sunlight irradiating the first plate's front side heats the plate and causes a portion of the water running down the opposite side to evaporate. The vapor condenses along the front side of the next adjacent plate, transferring heat to the flow of water on its opposite side and so on. Condensate generated along the front sides of the plates is separately collected at the bottoms of the plates.
To improve the efficiency of thin-film distillers, rotary evaporators have also been designed. For example, U.S. Pat. No. 4,731,159 to Porter et al., entitled EVAPORATOR, is directed to a rotary type evaporator having a plurality of horizontally stacked annular plates that are disposed within a housing and mounted for rotation about a central shaft. The ends of alternating pairs of plates are sealed to define sealed spaces. Each sealed space includes two inner plate surfaces facing each other and two outer surfaces, each of which is opposite a respective inner surface. The sealed spaces, moreover, are interconnected by a series of orifices and washers disposed between adjacent outer plate surfaces. A liquid to be distilled is introduced into the stack of rotating annular plates and enters each of the sealed spaces through an inlet port. As the liquid enters the spaces, it flows along the opposing inner surfaces of the space. A condensable vapor is introduced into the housing and is thus free to flow around the outer surfaces of the plates. The vapor is not, however, able to enter the sealed spaces. Since the liquid in the sealed spaces is at a lower temperature than the vapor, the vapor condenses along the outer surfaces of the plates. The condensate is thrown off of the rotating plates, collects inside the housing and is removed through an outlet port located in the bottom of the housing. Condensation of the vapor also transfers heat across the plates to the liquid, thereby causing a portion of the liquid in the sealed spaces to evaporate. The vapor exits the sealed spaces through the liquid inlet ports and is removed from the top of the housing. Any non-evaporated liquid remaining in the spaces flows upwardly along the sealed spaces through the corresponding orifice/washer arrangements and is also withdrawn from the top of the evaporator.
Although it may provide some advantages, the design of the '159 evaporator presents a substantial risk of contamination of the condensate by the liquid being evaporated and is thus not suitable to generating potable distilled water. In other words, with the evaporator of the '159 patent, the unsafe water which is being distilled is capable of mixing with and thus contaminating the distillate. For example, a leak at any of the sealed spaces would allow liquid from the sealed space to enter the housing and mix with the distillate being collected therein. The likelihood of such an occurrence, moreover, is not insignificant due to the corrosive attributes of some water sources and the high number of orifices and washers required to provide fluid communication between the various sealed spaces of the evaporator of the '159 patent.
It is also known to provide those plate surfaces on which liquid is evaporated with some type of hydrophilic treatment. That is, these plate surfaces are ideally treated to have a strong affinity for the liquid being evaporated, thereby causing the liquid to adhere to the entire plate surface (rather than simply forming narrow streams). Numerous techniques are known to provide hydrophilic properties to thin metal plates. The '159 patent, for example, notes that its plates may be chemically etched or sand-blasted. In addition, simply allowing copper plates to oxidize provides some hydrophilic effects. Other techniques include applying organic films to the plate surfaces. With most of these techniques, the plates are treated individually and then assembled together to form the distiller. During the assembly process, however, the hydrophilic treatment degrades, often substantially, due to the high temperatures required to assembly the treated plates. For example, conventional soldering and braising techniques generate temperatures on the order of 450.degree. F.
Multiple-effect distillation systems are also known. U.S. Pat. No. 2,894,879 to Hickman, entitled MULTIPLE EFFECT DISTILLATION, discloses a distiller having fifteen vertically arranged effects. Each effect includes a rotating evaporator section and an associated condenser section. The liquid to be distilled is supplied to the evaporator section of the first stage, which is located at the top of the distiller. A heat source, such as steam, is similarly provided to the condenser section of the first effect, in order to evaporate at least a portion of the liquid. The vapor generated in the evaporator section of the first effect is then transferred to the second effect condenser section where it is used to heat liquid left over from the first effect that is likewise provided to the evaporator section of the second effect. The distillate generated within the condenser section of the first effect is also transferred to the condenser section of the second effect. This process is repeated at each effect of the distiller. The distillate accumulated from each of the effects is then removed from the system. To achieve the desired flow among the effects, the distiller of the '879 patent includes numerous rotating tubing segments that are used to interconnect the various evaporator and condenser sections and to spray liquid onto the surface of the evaporator sections. Accordingly, the manufacturing and assembly costs of the system are relatively high. Furthermore, any leaks of liquid in the evaporator sections will contaminate distillate being collected in the adjacent condenser sections. The existence of any such leaks, moreover, would be extremely difficult to detect.
Vapor compression distillers, which can be more efficient than conventional distillers, are also known. The underlying principle of vapor compression distillers is that, by raising the pressure of a vapor (e.g., steam), its saturation temperature also rises. In the vapor compression distiller, vapor produced in an evaporator is removed, compressed (raising its saturation temperature) and returned to the evaporator, where it condenses, producing a distillate. Furthermore, the heat of vaporization that is given off as the vapor (having a raised saturation temperature) condenses is used to heat (and thus evaporate) the liquid being distilled. Large-scale vapor compression distillers using powerful centrifugal compressors can produce hundreds of gallons of distilled water a day. These distillers, however, do not scale well, as the operating costs associated with the centrifugal compressor make them impractical for installations that require only tens of gallons of distilled water a day.